Neuromodulatory approaches for chronic stroke patients are limited. Transcranial direct current stimulation (tDCS) has shown the potential to improve motor deficits in this population; however, its effects have not been consistent in randomized studies to date, limiting widespread adoption. A critical gap in our knowledge is a detailed understanding of how tDCS affects motor cortical oscillations, which are important in guiding voluntary movement. Our long-term goal is to develop effective neuromodulation-based therapeutic systems in chronic stroke patients based on a better understanding of how neuromodulation of cortical signals can improve recovery of motor behavior in this population. In a previous study, we recorded subdural electrocorticography (sECoG) in akinetic-rigid Parkinson?s disease (PD) patients undergoing DBS surgery, and observed significant modulation of motor cortical oscillations in relation to an arm-reaching task. Thus, changes in cortical oscillations supported improved motor performance in this group. Based on these results, our central hypothesis is that modulation of motor cortical oscillations both prior to and during movement may be one mechanism by which tDCS promotes recovery after chronic motor stroke. To test this hypothesis, in PD patients undergoing DBS surgery, we will measure cortical beta (13-30 Hz) and broadband gamma (70-200 Hz) oscillations during a cued arm-reaching task (Aim 1) and a motor imagery task (Aim 2) before and after anodal tDCS activation of primary motor cortex. In these patients, simultaneous sECoG and EEG will be performed. In order to ensure that our findings in PD will be directly translatable to a stroke model, in Aim 3 we will collect pilot data using combined tDCS and EEG in a cohort of chronic stroke patients performing the same arm task as PD patients in Aim 1. Aims 1 and 2 will be performed in the operating room, while Aim 3 will take place in the NI Core EEG lab. We anticipate that tDCS will modulate motor cortical oscillations in a way that biases movement planning and initiation in both populations. This proposal to combine tDCS with sECoG and EEG during neurosurgery is novel. If our hypotheses are confirmed, the findings may have use in developing a closed-loop form of tDCS for stroke recovery, for which we will have pilot data (from Aim 3) to inform such an approach. Furthermore, the measurement of EEG in both PD and chronic stroke patients, using an identical experimental paradigm and recording modality, ensures that the results will be directly interpretable between both models. Finally, our sECoG recordings will provide detailed spatial and frequency domain (above 70 Hz) information not captured by EEG. In the future, these results may potentially set the stage for an invasive system in chronic stroke patients based on sensing and responding to pathological oscillations, as is already commercially available for epilepsy patients and currently in development for PD.